Objectives: (a) To develop, standardize, and evaluate sensitive and specific assays for antibodies to Bordetella pertussis antigens. (b) To serve as a refer-ence laboratory for pertussis serology by writing reference methods, procuring reference reagents, training visiting personnel, and performing comparative test-ing with outside laboratories. For all assays listed below, protocols have been distributed, visitors were trained, and comparative testing with outside laboratories has been performed. (1) Microagglutination assay: Because the supply of US Reference Antigen for Pertussis Microagglutination Assay, Lot 1 was nearly depleted, production of Lot 2 was initiated and completed. The bacterial cells were lyophilized in vials, testing was performed to demonstrate the equivalence of Lots 1 and 2, and distribution of Lot 2 has begun. (2) CHO-cell neutralization Assay: Stained and dried demonstration plates have been prepared and are available for distribution to collaborators. (3) ELISA Assays: (a) Although Reference Pertussis Antisera (Human), Lots 3 and 4 have been proved to be excellent reference sera for IgG assays, they are suboptimal for IgA assays. Therefore 1500 vials of Reference Pertussis Antiserum (Human) Lot 5 were prepared specifically as a reference for IgA assays. (b) A number of collaborative assays have been conducted with outside laboratories to assess the within laboratory reproducibility and between laboratory comparability of a standardized ELIZA procedure. In addition to the Laboratory of Pertussis, two national control laboratories, one commercial clinical laboratory, two university laboratories and four vaccine manufacturers were included in the study. (c) Protocols and reagents for the standardized ELISA procedure were distributed to more than twenty laboratories. (d) Evaluation of the new ELISA calculation program continues.